Search systems provide a way for users to access a knowledge database that stores a large number of documents. A user seeking an answer to a specific question, such as a customer using a web portal or a customer service agent responsible for answering questions from the general public, enters a search query describing the question to the search system. The search system includes a search engine that returns a set of ranked documents in response to the search query. The documents are ranked according to a predicted relevance to the search query. The quality of the user's experience is related to the user's perceived relevance of the documents to the user's question. To this end, knowledge providers often “tune” search engines to return documents in response to a search query that are more relevant to the user's question than the documents returned by the original (i.e., “un-tuned”) search engine.